The present invention relates to a device for end sealing in a chambered doctor blade device, in the following also referred to as chambered doctor blade, for a printing unit, of the type defined in the preamble to appended claim 1. In addition, the invention relates to a chambered doctor blade having at least one such end sealing device.
Chambered doctor blades are extensively used in rotary-printing units, especially flexoprinting units, for applying ink, lacquer, adhesive or the like to a rotatable cylinder included in the printing unit. In a flexoprinting unit, the chambered doctor blade serves to ink a screen roller. This occurs by filling the cells or recesses of the roller with ink by means of the chambered doctor blade. Such a chambered doctor blade device is disclosed in, for example, WO 93/24328. Chambered doctor blades of this type comprise an elongate frame with two elongate doctor blades which are arranged alongside the roller in such manner that the longitudinal axis of the chambered doctor blade is parallel with that of the roller. The chambered doctor blade defines an elongate chamber which holds, for example, ink. When applying ink or the like to the circumferential surface of the roller, the chambered doctor blade is applied against the same. The function of each doctor blade changes with the direction of rotation of the roller. One of the doctor blades, the wiping doctor blade, wipes off excess ink while the other merely has a sealing function.
In the above construction, seals are required at the ends of the chambered doctor blade in order to laterally enclose the liquid in the chamber. Such end seals, which are adapted to seal internally against the chambered doctor blade and against the circumferential surface of the rotating cylinder, advantageously comprise flexible sealing means made of, for example, rubber or foamed plastic material. The drawback of these sealing means is that they are thick and thus abut against a significant part of the doctor blades, which means that ink can be accumulated and dry between the sealing means and the edges of the doctor blades, which in turn causes a deteriorated function of the doctor blades. Moreover this type of sealing means has a short service life.
To cope with some of the above difficulties, end seals, which comprise a considerably thinner sheet of semi-rigid plastic material, such as polytetrafluoroethylene (PTFE, TEFLON(copyright)) have been developed. To establish a satisfactory seal against all surfaces, an accurate fit of the Teflon(copyright) sheet to the surfaces is necessary. Consequently, great dimensional accuracy is necessary in the manufacture of both the Teflon(copyright) sheet and its holder. Careful handling of the Teflon(copyright) sheet and a certain degree of skilfulness of the fitter are necessary to provide exact positioning when mounting the end seal. One more problem is the fact that there is a risk of folds forming in the Teflon(copyright) sheet when being applied against the roller. In case of folds, ink or the like will leak out of the chamber.
The problem of leakage caused by folding can be compensated for by letting an end seal comprise two or more spaced-apart thin sealing sheets to form a so-called labyrinth packing. An end seal of this kind, which in fact functions well in practice, is disclosed in DE-U-94 05 883.
A problem of end seals which comprise a plurality of thin sheets made of e.g. Teflon(copyright), however, resides in a more complicated construction. The fact that the end seal consists of many parts and comprises a plurality of Teflon(copyright) sheets also means that it will be expensive. Moreover, today""s quick-drying inks will have time to dry in the gap forming between the sealing sheets in the application against the roller, which means that the labyrinth effect will partly be lost.
In view hereof, the object of the present invention is to provide an end sealing device which is improved in relation to prior-art and which at least partly solves the above problems.
This object is achieved on the one hand by means of a device having the features recited in independent claim 1 and, on the other hand, by means of a chambered doctor blade having the features as defined in claim 20.
Additional distinguishing features and advantages of the invention are apparent from the dependent claims and the following description.
By the end seal according to the invention comprising two sealing sheets of which the first and the second sealing sheet are mutually arranged to seal against different portions of the inside of the space which is defined by the chambered doctor blade and the cylinder and which may contain ink or the like, a plurality of inconveniences caused by prior-art technique can be obviated. The first sealing sheet seals against the inside of the frame of the chambered doctor blade which in the operative position of the chambered doctor blade is essentially stationary, while said sealing sheet is located at a distance from the circumferential surface of the movable cylinder. The second sealing sheet seals against the doctor blades and, in the operative position of the chambered doctor blade, against the circumferential surface of the movable cylinder. Each sealing sheet thus need not seal against the inside of the space along its entire circumference. This means that the requirements as to dimensional accuracy will be lower, which enables lower costs in connection with the manufacture of the sealing sheets. Moreover the mounting work is facilitated since the accuracy need not be very high.
One more advantage of the inventive end seal is that, for example, the material and thickness of the sealing sheets can be optimised based on the requirements placed on the respective sheets. Thus, for example materials for the first sealing sheet, which preferably seals against essentially stationary parts in the printing unit, can be selected independently of the material which is considered suitable for the second sheet, which seals against the doctor blades and the movable cylinder.
The first sealing sheet is not subjected to mechanical wear to the same extent as the second sheet. The inventive end sealing allows that only the second sheet be exchanged.
By letting the second sealing sheet have a sealing lip projecting relative to the first sealing sheet and adapted to be applied against the circumferential surface of the cylinder, there is a decreased risk of the first sealing sheet contacting the circumferential surface of the cylinder also when the doctor blades have been worn down.
Thanks to the sealing lip, applicable against the cylinder, of the second sealing sheet being adapted to be curved over an edge surface, facing the cylinder, of the first sealing sheet, the pliability of the second sealing sheet relative to the cylinder is improved by obtaining an elastic support from the first sealing sheet. Moreover the first sealing sheet is protected from contacting the movable parts of the construction, which means that it is not subjected to a considerable degree of mechanical wear. Furthermore owing to the curve of the sealing lip, a satisfactory seal against the circumferential surface of the cylinder is maintained also if the sealing lip is worn.
By forming, according to a preferred embodiment, a recess in the sealing lip of the second sealing sheet so as to give the sealing lip a curved shape, better application against the circumferential surface of the cylinder and, thus, a more efficient sealing function are rendered possible. Furthermore the risk of folding of the sealing sheet can be eliminated. The mechanical wear on the second sealing sheet can be reduced by letting the recess of the sealing lip have a radius of curvature corresponding to the radius of the cylinder.
Preferably, the edge surface, facing the cylinder, of the first sheet is allowed to have a radius of curvature corresponding to that of the sealing lip of the second sealing sheet, thereby achieving improved cooperation between the sealing sheets.
The fact that the second sealing sheet, in the operative position of the chambered doctor blade, seals essentially merely against the movable parts of the construction, i.e. against the circumferential surface of the cylinder and a portion, closest to the cylinder, of each doctor blade; means that the application against the cylinder causes a xe2x80x9csingle curvexe2x80x9d of the sheet. This minimises the risk of folding of the sheet, and leakage of ink or the like through the end seal can be prevented.
Owing to the fact that the edges of the first sealing sheet are located at a distance from the portions against which the second sealing sheet seals, the risk is minimised that dried ink or the like is accumulated under the wiping doctor blade and thus deteriorates its function.
The preferably plane sealing sheets advantageously abut against each other for improved cooperation.
An advantage of making the first sealing sheet of a compressible material is that a satisfactory seal against the inside of the chamber can be established even if the dimensions of the sheet are not exact. Consequently, this means lower requirements as to dimensional accuracy in manufacture and accuracy in the mounting work.
The first sealing sheet is advantageously made of a foamed polymer material, for instance polyethylene, thereby keeping down the costs of the end sealing.
An advantage of making the second sealing sheet, which seals against the-circumferential surface of the cylinder and the portion, closest to the cylinder, of each doctor blade, of a yieldable and semi-rigid material is that the sheet can be made thin. Yieldable materials permit preforming of the sealing sheet for an improved sealing function. Such preforming can, for example, be made manually. If the sheet is preformed, the risk of leakage owing to folding can be further reduced. Moreover, mechanical wear on a suitably selected yieldable material usually occurs more slowly than on, for instance, a compressible material.
If the second sealing sheet is made of a polymer material, preferably polytetrafluoroethylene (PTFE, for instance TEFLON(copyright)), a wear-resistant and formable sheet is obtained for sealing abutment against the circumferential surface of the cylinder and against the portion, closest to the cylinder, of each doctor blade.
If a first sealing sheet is selected, which in a preferred embodiment is considerably thicker, about 3-35 times, than the second sealing sheet, improved cooperation between the two sealing sheets is rendered possible since the first sheet affords the second sheet a better support.
An advantage of letting the first sealing sheet be relatively thick, about 3-15 mm and preferably about 5 mm, is that it allows improved sealing against the essentially stationary parts of the chambered doctor blade since the abutment surfaces will be greater and the risk of folding decreases.
By making the second sealing sheet as a thin sheet or foil, having a thickness of less than below about 1.1 mm and preferably 0.3-0.6 mm, it merely occupies a smaller area of a small extent in the longitudinal direction of the cylinder. This minimises the risk of deterioration of the sealing of the doctor blades against the cylinder caused by ink or the like accumulating behind the wiping doctor blade. Also the force applied by the second sealing sheet to the doctor blade is minimised, which results in less wear on the doctor blade at the end sealing.
An advantage of letting the holding means of the end seal being designed in such manner that the sealing sheets can be clamped against each other is that improved cooperation between the sealing sheets is achieved. Furthermore a simple construction of the end sealing device with a small number of elements is obtained, which facilitates, for instance, cleaning thereof.